In WCDMA, the open loop downlink transmission diversity employs a Space Time block coding based Transmit Diversity (STTD), see 3GPP TS 25.211, “Physical channels and mapping of transport channels onto physical channels (FDD)”, which is based on the Alamouti Space-Time Block Code (STBC), see S. M. Alamouti, “A simple transmit diversity technique for wireless communications”, IEEE Journal on Selected Areas in Communications, Volume 16, Issue 8, October The STTD encoding is optional in UTRAN (UMTS Terrestrial Radio Access Network) UMTS is the Universal Mobile Telecommunications Systems and STTD support is mandatory in the User Equipment (UE).
Space-time block coding is a technique used in wireless communications to transmit multiple copies of a data stream across a number of antennas and to exploit the various received versions of the data to improve the reliability of data-transfer. The first STBCs were designed for a two transmitting antenna, which is a rate-1 code. It takes two time-slots to transmit two symbols, and there are two copies of each symbol transmitted.
In FIG. 1 such a diversity scheme is depicted as a baseband representation. The scheme is designed for two transmitting antennas and one receiving antenna. At a given symbol period, two signals are simultaneously transmitted from the two antennas. The signal transmitted from antenna zero is denoted by s0 and from antenna one by −s1*. During the next symbol period signal s1 is transmitted from antenna zero, and signal s0 is transmitted from antenna one where * is the complex conjugate operation. This sequence is shown in Table 1 below.
TABLE 1The encoding and transmission sequence for the two-branchtransmit diversity schemeAntenna 0Antenna 11st symbol periods0−s1*2nd symbol periods1s0*
In Table 1 the encoding is done in space and time (space-time coding). The encoding, however, may also be done in space and frequency. Instead of two adjacent symbol periods, two adjacent carriers may for example be used for the two symbol periods (space-frequency coding).
Moreover in the U.S. Pat. No. 6,754,253, a RAKE receiver structure is proposed for the transmit diversity in CDMA system where the mobile terminal includes a first rake receiver matched to a first transmitting antenna and a second rake receiver matched to a second transmitting antenna.
Also, the generalized RAKE (GRAKE) receiver, see G. E. Bottomley, T. Ottosson, and Y.-P. E. Wang, “A generalized RAKE receiver for interference suppression,” IEEE J. Select. Areas Commun., vol. 18, pp. 1536-1545, August 2000 is a good means for suppressing the colored interference from the own base station in a CDMA system. The GRAKE receiver is also extended to MIMO system. see S. J. Grant, K. J. Molnar, and G. E. Bottomley “Generalized RAKE receivers for MIMO systems”, VTC 2003-Fall. 2003 IEEE 58th, Volume I and Wang Hai, Miao Qingyu, “GRAKE2 modeling”, EAB-06:039491, pB. Also GRAKE for STTD in WCDMA system is discussed in Y.-P. Eric Wang, G. E. Bottomley, and A. S. Khayrallah, “Transmit Diversity and Receiver Performance in a WCDMA System”, submitted to globecom 2007.
In FIG. 2 a typical RAKE receiver structure 200 is depicted. In the existing structure 2 receivers are used, the first receiver detects the signal in the first symbol period and the second receiver detects the signal in the second symbol period. In the end, the signal is combined. Hence in the existing receiver structure, the receiver is configured to first obtain the channel coefficient h with channel estimation. The weighting factors w1* for the first symbol period is then obtained using the channel coefficient h, the power, the interference and noise in the first symbol period. The signal received by a first antenna 201 in the first symbol period is fed to the taps 205 of the first receiver of the RAKE/GRAKE receiver 200. The signal is despread in despreading modules 209 for each tap 205. Each despreaded signal is then multiplied with the corresponding weighting factors w1* for the first symbol period. A received signal for the first symbol period is then formed in a receiver 215. Next, the weighting factors w2* for the second symbol period are calculated with the channel coefficient h, the power, the interference and noise in the second symbol period. The detected signal in the second symbol period with the weighting factor w2*for the second symbol period is then obtained in the same manner as the signal in the first symbol period. Hence, the signal associated with the second symbol period received by a second antenna 203 is despread in despreading modules 209 for each tap 207. Each despreaded signal is then multiplied with the corresponding weighting factors w2* for the second symbol period. A received signal for the second symbol period is then formed in a receiver 217. Finally, the received signal in the first symbol period and the received signal in the second symbol period are combined to for a combined received signal in a combiner 219.
There is a constant demand to improve the efficiency in existing telecommunications systems and to utilize resources in a more cost efficient way. Hence, there exist a need for a method and a system that is able to improve over existing telecommunications systems.